A quantum channel is a channel for carrying quanta such as photons or electrons from a transmitter to a recipient. A specific example of a quantum channel is an optical fiber. In order to generate quanta for transmission over a quantum channel, a Poisson source is typically used. The Poisson source is characterized by the emission of a variable number of quanta per interval of time (or “pulse”), where the probability distribution of the number of quanta per pulse over a large number of pulses resembles that of a Poisson random variable. Through external regulation of the Poisson source, it is possible to control the mean number of quanta per pulse (denoted λ).
Quantum cryptography is a field of technology devoted to the study of methodologies that exploit certain enhanced security possibilities afforded by the quantum nature of the aforementioned quantum channel. One such methodology is the BB84 protocol, described in C. H. Bennett and G. Brassard, “Quantum Cryptography: Public Key Distribution and Coin Tossing”, Proceedings of IEEE International Conference on Computers Systems and Signal Processing, Bangalore, India, December 1984, pp. 175-179, hereby incorporated by reference herein. Assuming the validity of certain assumptions, the BB84 protocol allows communicating parties (referred to in the literature as “Alice” and “Bob”) to detect when photons have been intercepted or otherwise tampered with by an intermediate party (referred to in the literature as “Eve”).
One of the assumptions required for effectiveness of the BB84 protocol and other existing quantum cryptographic methodologies is that there be virtually zero probability of finding two or more photons in a single pulse. To achieve this condition using the aforementioned Poisson source, the value of λ (i.e., the mean number of photons per pulse) needs to be significantly reduced, often to a point where only every tenth or hundredth pulse, on average, contains photons (and hence can carry information). Thus, even with back-to-back pulses of a duration as short as several nanoseconds (ns), one will appreciate that the data rate on the quantum channel can drop to a few hundred bits per second or less, depending on prevailing conditions (e.g., loss, dispersion, detection efficiency, etc.).
In view of the above, it is clear that in order to guarantee the effectiveness of conventional quantum cryptographic methodologies, the data rate of the quantum channel will be artificially constrained to a low value. It is thus not surprising that the applications which most commonly use conventional quantum cryptographic methodologies involve securely transmitting very small amounts of secret information. For example, this can include the distribution of a secret or private key, which is then used to encrypt (over a classical channel) larger amounts of information requiring secure transmission.
While it would be desirable to achieve higher data rates over the quantum channel, one cannot merely increase the value of λ at the Poisson source without sacrificing the security benefits of conventional quantum cryptographic methodologies. This is because even though increasing λ will cause a greater percentage of pulses to contain photons (and hence can carry information), a significant number of these pulses will contain more than one photon. This violates the aforementioned condition for effective use of conventional quantum cryptographic methodologies, which requires that there be virtually zero probability of finding a pulse that contains more than a single photon. The net effect of this violation is that during those pulses that contain multiple photons, eavesdropping attacks on the quantum channel may go undetected by conventional quantum cryptographic methodologies.
Thus, it would be advantageous to increase the bandwidth of the quantum channel while continuing to be in a position to detect eavesdropping activities that may occur on the quantum channel. This would enable not only a greater speed of secret/private key distribution, but also would enable generalized higher-bandwidth exchanges to take place in a secure manner.